Industry has come under strict regulations to limit the level of contaminants in discharged wastewater. Particularly restrictive discharge regulations have been adopted for contaminants harmful to humans and aquatic organisms such as heavy metals; for example, copper, lead, zinc, cadmium and nickel, and certain non-metals such as arsenic and selenium. Consequently, a number of processes have been proposed to reduced the heavy metal content of wastewater to meet increasingly stringent discharge limits.
One common practice is to precipitate the bulk of the heavy metal contaminant as its metal hydroxide. Metal ions such as copper and lead are easily precipitated in this way, but the minimum concentration that can be obtained is limited by the finite solubility of the hydroxide complexes. The resulting effluent from the hydroxide precipitation may be treated with a metal scavenging agent to remove any trace metal contaminants to meet discharge regulations. These agents may be precipitants, adsorbents, or metal specific ion exchange resins. The metal scavenger precipitants may also be effective when added in the same step as the hydroxide precipitation. Typical compounds utilized as precipitating scavenging agents include sulfides, (thio)carbonates, alkyl dithiocarbamates, mercaptans, and modified natural products.
Non-metals such as selenium and arsenic are not easily precipitated with hydroxide. These ions are typically co-precipitated with metal salts such as ferric sulfate. Traditional precipitant metal scavenging agents are usually ineffective in removing these ions.
The prior art scavenging agents have other limitations. The metal thiocarbonates, sulfides, mercaptans, and thiocarbamates form fine flocs which are not conducive to settling and typically require the use of a flocculation agent and/or filtration to achieve efficient removal. Prior art polydithiocarbamates are characterized as having limited water solubility, which limits the degree of functionalization possible. In addition, a number of the prior art scavenging agents are themselves very toxic and care must be taken to ensure that they are not present in the discharged wastewater.
There exists a need, therefore, for a less toxic scavenging agent that forms larger, faster settling floc than the prior art compounds. It is also desirable that this material be water-soluble so it can be utilized in existing clarification facilities, avoiding the need for capital investment in resin-bed apparati or other specialized equipment.